Oleaginous plants contain many fatty acids, the most widespread of which are: palmitic acid, stearic acid, oleic acid, linoleic acid, linolenic acid, erucic acid, etc. Also among the fatty acids of oleaginous plants are fatty acids that are functionalized, in particular by means of one or more alcohol functions or of an epoxide function. The most widely known are the following fatty acids:                ricinoleic acid (12-hydroxy-cis-9-octadecenoic acid), predominantly present in castor oil (between 85% and 90%) extracted from seeds of the castor oil plant (Ricinus communis); this hydroxy acid is also present at a content of approximately 18% in the oil extracted from Jatropha gossypiifolia seeds or in that from Hevea brasiliensis;         lesquerolic acid (14-hydroxy-11-eicosanoic acid) extracted from seeds originating from species of the Lesquerrella genus, at a content of approximately 70% in the oil extracted from seeds originating from species of the Lesquerrella genus, in which there are also two other hydroxy acids: densipolic acid (12-hydroxy-9,15-octadecadienoic acid) and auricolic acid (14-hydroxy-11,17-eicosadienoic acid);        beta-dimorphecolic acid (9-OH-18:2Δ10trans,12trans) present in the oil extracted from seeds of Dimorphotheca pluvialis, and its isomer coriolic acid (13-hydroxy-9,11-octadecadienoic acid) present at a content of approximately 70% in the oil extracted from seeds of Coriaria myrtifolia;         kamlolenic acid (18-hydroxy-9,11,13-octadecatrienoic acid) present in the oil extracted from the seeds of the Kamala tree (Mallotus phillipinensis);        coronaric acid (9,10-epoxy-cis-octadec-12-enoic acid) present in sunflower (Chrysentemum) oil, or at a content of approximately 91% in the seeds of Bernardia pulchella;         vernolic acid (cis-12,13-epoxyoleic acid) present at a content ranging from 60% to 75% in the oil extracted from the seeds of Euphorbia lagascae or plants of the Vernonia genus, and also the hydrogenated homologs of these unsaturated fatty acids.        
Oleaginous fatty acid esters are conventionally obtained by transesterification of the refined oil obtained from seeds by pressing, followed by extraction using an organic solvent such as hexane or acetone.
This set of processes is laborious, since it comprises many physicochemical transformation steps. Moreover, the resulting hydroxylated fatty acid esters have a high production cost.
Patent application EP 1 119 600 describes a method for producing fatty acid esters from oleaginous seeds, in an alcoholic medium optionally supplemented with another organic solvent, for example a ketone or an aliphatic hydrocarbon, in the presence of a basic catalyst. This method is advantageous since it makes it possible to reduce the fatty acid ester production costs, and has very good yields of these products.
However, this method has the following drawbacks i) and ii):    i) This technology is not simply transposable to the seeds of oleaginous plants, the extracted oils of which have contents of at least 10% in terms of functionalized fatty acids, in particular hydroxylated fatty acids, such as ricinoleic acid or lesquerolic acid. One of these plants is the castor oil plant. Castor oil comprises glycerol triricinoleate as the main component. Ricinoleic acid is a hydroxylated fatty acid. No other known natural oil contains such a high proportion of hydroxylated fatty acids. It is this characteristic glyceride composition which distinguishes castor oil from any other vegetable fats and oils and it is this which gives it its notable physical and chemical properties. Castor oil, which is a non-drying oil, thus has the highest viscosity number and density of all the natural oils. These properties are due in particular to the hydrogen bonds formed between the hydroxyl groups. Moreover, methanol is much more soluble in castor oil, but unfortunately so is glycerol. It is these properties which give castor oil a very particular behavior during chemical reactions and which mean that the known methods for transesterification of “ordinary” vegetable oils cannot quite simply be transposed to castor oil (N.B., the term “ordinary oil” is intended to mean nonfunctionalized oils extracted from oleaginous plants such as sunflower, rape, soya, peanut, olive tree, sesame, safflower, coconut or palm).    ii) In addition, this method does not make it possible to selectively extract the functionalized, in particular hydroxylated, fatty acid esters from the mixture of fatty acid esters obtained by means of the transesterification reaction. However, it is desirable to have a fraction enriched with hydroxylated fatty acid esters, containing, for example, methyl ricinoleate (or methyl 12-hydroxy-cis-9-octadecenoate), since such a fraction represents the starting point in the production of 11-aminoundecanoic acid, a constituent monomer of Rilsan® 11, which is a polyamide with exceptional physical properties, developed by the applicant. During the production of 11-aminoundecanoic acid, methyl ricinoleate is subjected to gas-phase thermal cracking. To this effect, it must contain a minimum amount of glycerides, i.e. of tri-, di- and monoglycerides, since these products are very difficult to vaporize, and often break down before vaporization, which results in a lowering of the selectivity of the cracking. Similarly, the methyl ricinoleate must contain a minimum amount of ricinoleic acid, which is itself also difficult to vaporize.
Document U.S. Pat. No. 7,112,229 describes a method for obtaining fatty acid esters for a biodiesel application. The method is carried out starting from oil-rich oleaginous plant seeds. This method comprises a prior step of processing the seeds, during which:                the holes are removed from the seeds and the seeds are then cleaned using a vibrating sieve,        the cleaned seeds are dried so as to reduce their moisture content to less than 0.5% by weight.        
The dried seeds are introduced into a reactor with anhydrous alcohol; this heterogeneous mixture is converted into a homogeneous suspension by means of an agitator. It is only at this moment that a basic catalyst is introduced into the reactor. This reaction mixture is then heated for 30 to 90 minutes at a temperature of 30 to 78° C., resulting in transesterification of the triglycerides to give esters with a high conversion rate, of between 98% and 100%. The applicant has carried out tests according to the conditions given in the only example described in that document, but has not succeeded in obtaining the same results (see example 5 and appended FIGS. 2 and 3). It cannot therefore be considered that this technique is currently mastered by those skilled in the art.
A method for the selective extraction of esters of a hydroxylated acid is described in the publication by Tassignon P. et al., Chem. Phys. Lipids 71 (1994), 187-196. That document describes the obtaining of the methyl ester of dimorphecolic acid by transesterification of the crude oil extracted from Dimorphotheca pluvialis seeds in the presence of methanol and of a basic catalyst. The transesterification reaction is followed by an extraction step by means of aqueous methanol/hexane (1:1) or acetonitrile/hexane mixtures. The data given in table 1 show that the separation by means of aqueous methanol/hexane mixtures is better than that carried out by means of an acetonitrile/hexane mixture. This method remains very laborious, efficient separation requiring 4 funnels and 14 stages for separation of the two-phase mixtures. Furthermore, the liquid-liquid extractions described in paragraph 2.12 require the consumption of a large amount of solvents: 1970 ml of hexane and 3360 ml of methanol solution are required to treat 70.3 g of methyl ester and to recover 37.2 g of oil containing 95% of dimorphecolic acid methyl ester. Such amounts of solvent are not compatible with an industrial-scale application of the method for selective extraction described.
The present invention proposes to remedy these drawbacks by means of a method which makes it possible to selectively extract functionalized fatty acid esters. The term “functionalized fatty acid” is intended to mean, in the context of the invention, any fatty acid bearing in particular one or more alcohol functions or an epoxide function.